1. Field of the Invention
The present invention relates generally to telecommunication networks, and more particularly, to a frame selection and power control CDMA architecture for telecommunication networks.
2. Discussion of the Related Art
In cellular telephone systems, a service area is divided into cells, each of which may be further divided into sectors. Each cell is serviced by a single base station (BS) and all of the base stations are connected to a mobile switching center (MSC) (also referred to as a Mobile Telephone eXchange (MTX)) via a base station controller (BSC) and hardware links (e.g., T1, E1, optical fiber, or satellite links). A plurality of mobile units (i.e., subscriber units) can be connected to the MSC by establishing radio links with one or more nearby base stations.
FIG. 1 illustrates an exemplary MSC and Code Division Multiple Access (CDMA) system architecture 10. The MSC 12 is coupled to a Public Switched Telephone Network 14 (PSTN) or other public network. The CDMA system 10 includes a plurality of base station transceiver subsystems (BTSs) 16, each of which define respective cells 18. Each cell can have a coverage area radius on the order of approximately 1-6 miles, typical. Various components of mobile telephone systems are known in the art and thus only those components of mobile communications which are pertinent with respect to the present disclosure are disclosed and briefly described herein.
The system architecture of FIG. 1 further includes abase station controller (BSC) 20 having a router 22 (also referred to as a CDMA Interconnect System (CIS)) and a selector 24 (SEL). The CDMA router 22 provides a packet routing function and allows the MSC-CDMA system 10 to be fully connected, i.e., any component can communicate with any other component in the system. The router 22 thus routes packets and provides any necessary communication between, for example, any base station transceiver subsystem (BTS) 16, the selector (SEL) 24, or the MSC 12. The router is also capable of performing the broadcast of packets.
The selector (SEL) 24 handles an appropriate data formatting of voice data on the MSC-side and on the BTS-side of the selector. The SEL includes a selector subsystem including vocoder (voice coder/decoder) digital signal processors (DSPs) and call processing managements functions. The SEL 24 further receives control information from the MSC 12 via the router 22. In particular, the selector (SEL) provides the functions of voice coding, multiplexing, handoff, power control, and radio link setup. A voice coding component provides conversion of pulse code modulation (PCM) format voice received from a digital trunk controller (DTC) of the MSC 12 into the CDMA format frames destined for the BTS 16 using a suitable coding technique. The digital trunk controller (DTC) supports trunk terminations to the PSTN 14, furthermore, providing necessary processing and control functions between the MSC 12 and the PSTN 14. The voice coding component also converts the CDMA format frames from the BTS 16 into the PCM format voice for use in the other direction. A multiplexing component processes all the IS95 traffic frames to multiplex the voice, data and signaling information. The handoff component coordinates the communications between the subscriber unit 26 and multiple BTSs 16. The power control component of SEL 24 maintains the mobile transmit power at a desired power level. Lastly, the radio link setup component of SEL 24 is used during call setup for preparing the traffic channel on the BTS.
In further discussion of the above, the selector (SEL) includes a plurality of independent DSP units. For each mobile station, there is a single DSP unit assigned. Only one DSP is used per call originating to/from a given subscriber unit or mobile station. The unique identifier of the subscriber unit determines which DSP of the plurality of DSPs the mobile station is dedicated to.
In the instance of a mobile station traveling from a first cell to a second cell, the selector (SEL) receives one packet plus a soft handoff packet, to be further discussed below. The selector (SEL) performs some prescribed call management, including power control, selecting the best input signal packet of the one or more packets received, and sending the selected packet to the corresponding DSP (i.e., a selector card of the SEL interfaces with a corresponding DSP).
The base station transceiver subsystem (BTS) 16 provides the radio link between subscriber units (also referred to as mobile stations/mobile units) 26 and the MSC 12, wherein the BTS is located at a respective cell site. Located at the BTS or respective base station are the antennas, transmitter, receivers, power amplifiers, and interface hardware to support the link to the base station controller (BSC). Each base station provides a common air interface to the subscriber units according to the CDMA standard. For example, data from the subscriber unit 26 is converted to packets by the base station, and these packets plus additional control information are passed to the selector (SEL) 24 in the base station controller (BSC) 20 for further processing.
Each base station transceiver subsystem (BTS) thus corresponds to cell site equipment for the MSC-CDMA system and is used for performing various software functions. The BTS provides the IS95 air interface between the MSC-CDMA system and the subscriber unit. In the forward direction, the BTS accepts packets from the SEL and modulates the information on the RF carrier and transmits the packet. In the reverse direction, the BTS demodulates the RF back into packets, adds additional control information and then routes the packets via the router to the SEL for further processing. The major functions provided by the BTS software include: Over-the-air RF interface with the subscriber unit; additional over-the-air functions such as pilot, sync, paging, and access channels; call processing functions to control the subscriber unit operation over the paging and access channels, including short message services; control and management of BTS resources; and monitoring and configuration functions. BTS can either be integrated to include both digital processing and RF components, or can be distributed to allow for remote location of the RF equipment from centralized digital equipment.
Communication between a mobile station (MS) 26 and the PSTN 14 is carried out from a respective BTS or BTSs 16 to the router 22, from the router 22 to the selector (SEL) 24, from the SEL 24 to the MSC 12, and finally between the MSC 12 and the PSTN 14. Each BTS communicates with the router via a T1 (or E1) link 28. A T1 link is characterized by a communication rate of 1.54 megabits per second (Mbps) and an E1 link is characterized by a communication rate of 2 Mbps.
With the CDMA system 10, a mobile station 26 can begin a call in a first cell and subsequently travel into a second cell. While the mobile station is in the first cell, communication will occur between the respective BTS and the router via a respective T1 link. During a transition between the first cell and the second cell, it is possible for more than one communication to occur for a given transmission, i.e., from more than one BTS and T1 link to the router. This situation occurs when a mobile station is talking to more than one BTS and in which a signal is transmitted from each BTS. In addition, the voice communication signal is a compressed voice signal, further being transmitted in the form of packets. The packets are sent through a respective T1 link from a respective BTS to the router for ultimate delivery to a dedicated DSP within the selector (SEL) 24 for the given call. Prior to reaching the corresponding DSP, the packet goes to the selector (SEL) 24, wherein the selector 24 may receive multiple packets at any given time, each packet originating from a different BTS for a given call. The selector (SEL) 24 examines all packets received to determine which of the several packets for a given call to select for further handling. At this point in the process, the selected voice packet is still compressed. A selected packet is then sent to the DSP, where the DSP decompresses the compressed voice packet or message. The decompressed (or uncompressed) voice is then sent to the MSC. The size of an uncompressed voice packet is approximately 160 bytes. When in a compressed state, voice data is compressed to a size within a range on the order of thirteen to forty-five (13-45) bytes for each packet. In comparison to sending uncompressed packets of 160 bytes in size, when compressed data is sent on a T1 link and router, more compressed data can be supported than if not compressed.
In earlier cellular telephone technology, such as time division multiple access (TDMA), as a mobile unit traveled from one cell to another, the radio link between the mobile unit and the base station serving the first cell (source cell) had to be broken then replaced by a radio link between the mobile unit and the base station serving the second cell (targer Cell). In contrast, in a code division multiple access (CDMA) cellular telephone system, because the same frequency band is used for all cells and sectors, the first link need not be broken before connecting with the second link. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. A mobile unit thus need not switch frequencies when a call is transferred from one cell or sector to another. Additional details regarding the specifics of the CDMA cellular telephone environment are described in TIA/EIA/IS-95-A, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereinafter, CDMA standard), incorporated herein by reference in its entirety.
In the context of a cellular telephone system, xe2x80x9chandoffxe2x80x9d is the process of handing over a call from one sector (or cell) to another when a mobile unit (also referred to as a subscriber unit or mobile station) detects that acceptable communications with the other sector are possible. This occurs mainly when the mobile unit nears a sector boundary or the current communication link is weakened by radio frequency (RF) shadowing and another potential communication path from another sector is enhanced.
The term xe2x80x9csoft handoffxe2x80x9d is commonly used to refer to a handoff in which the mobile unit commences communication with a new base station without interrupting communication with the old base station, i.e., the call is maintained on both base stations. If there are three cells involved in the handoff, the call will be maintained by all three base stations. A xe2x80x9csofter handoffxe2x80x9d refers to a handoff in which the call is maintained on one base station for different sectors of the same cell. A hybrid form of the previously discussed types of handoff, referred to as a xe2x80x9csoft/softer handoffxe2x80x9d, results if there are two sectors from one cell and another sector from another cell involved in the handoff, in which case two base stations are involved. The terms xe2x80x9chandoffxe2x80x9d and xe2x80x9csoft handoffxe2x80x9d will hereinafter be used interchangeably to refer to all of the foregoing types of handoff.
Data that is currently available indicates that the amount of CDMA soft handoff is high. Typically, the soft handoff percentage in an MSC-CDMA system is approximately 50-to-70% of total system traffic. Despite its advantages, soft handoff still has a negative side effect due to its increased RF modems and backhaul bandwidth requirements (i.e., in order to transport the extra voice frames from base stations involved in soft handoff). For instance, with the current MSC-CDMA system, soft handoff activity requires the use of two or more packets on the T1 links to the CDMA router, one each from respective BTSs involved in the soft handoff. During soft handoff, the affected T1 links and the CDMA router voice call capacity is lessened or reduced. It would thus be desirable to alleviate the impact of these additional requirements.
In current CDMA systems, one disadvantage is that the voice call capacity of a T1/E1 line is smaller for higher percentage of soft handoff activity. If a T1/E1 line does not carry soft handoff traffic, then the T1/E1 line can support more calls on a same link. This is applicable (i.e., holds true) for fiber optic and satellite links, also.
In addition, a restriction in the CDMA architecture is that the CDMA system currently sends a voice packet or frame every 20 milliseconds. In designing any changes into the system, a variable delay (i.e., path delay) from the BTS to the selector (SEL) should not exceed 20 milliseconds, otherwise the voice communication is disrupted or out of sequence. In other words, the variation in delay between successive packets cannot exceed more than 20 milliseconds. For instance, if one packet took five milliseconds, another packet took 25 milliseconds, and a third one took only one millisecond, then what would happen is that the voice message would be received out of sequence. In such an example, the difference in delay between 25 milliseconds and one millisecond is greater than 20 milliseconds. As mentioned, the variable delay must be less than 20 milliseconds. This restriction is a result of the CDMA architecture and hardware limitation. Of the 20 milliseconds variable delay, currently some of the variable delay time is consumed by the T1 link, some consumed by the router, and some consumed by the selector (SEL), and what remains in approximately eight to nine (8-9) milliseconds. The important point is the variation of the delay. If the delay is 40 milliseconds, and the delay varies between 30 and 50 milliseconds, then it is okay. If the delay is 100 milliseconds, and it varies between 90 milliseconds to 110 milliseconds, then it is okay. What is important is that the delay (from 1 packet to the next) between packets cannot vary by more than 20 milliseconds, otherwise the CDMA system will not be able to handle the call (i.e., the call will be dropped). The CDMA system can adjust for any variations in delay of not more than 20 milliseconds.
Despite the benefit of soft handoff, current CDMA architecture satellite systems suffer from limitations such as backhaul delay. Backhaul delay is an important characteristic parameter for satellite based systems. That is, backhaul and propagation delays are a concern with respect to transmission from a BTS to a satellite to a router. Propagation delay is equivalent to the length of travel divided by the speed of the medium (speed of light for air). Backhaul delay is the sum of propagation delays plus device delay(s) (such as repeater(s)) over the link transport. For a satellite, backhaul delay is equal to the orbit length divided by the speed of light. In addition to backhaul delay, operating costs for a T-1 link are important, especially in connection with terrestrial cellular systems.
With current CDMA systems, a maximum tolerable backhaul delay is limited to approximately 8-9 milliseconds. This backhaul delay number is likely to decrease as other features are used in the CDMA systems (e.g., support for inter-system soft handoff). In addition, the transport of 8-9 milliseconds will not be enough for future technology applications. For establishing a satellite link, i.e., from a BTS to a satellite and onto the router, there is a required delay on the order of 14 milliseconds (excluding delay within the satellite itself). In such an instance of establishing a satellite link, if there is an island which would require the use of one satellite link to another satellite link, then the 8-9 milliseconds variable delay restriction would be violated, and the communication would not be possible. One reason is that the packet selection occurs subsequent to the router. If the selector has time to wake-up and it does not see any packets coming, what it will do, it will pass a silent tone (i.e., a silent interval). If no packets are received or if silent tones persist for more than a prescribed duration, then the call will be dropped.
The typical round trip (two satellite hops) delay for a satellite varies between approximately 8-14 milliseconds depending upon a particular satellite orbit, which prohibits the use of backhaul frame transport (i.e., BTS-to-BSC) over satellite links with the current MSC-CDMA product architecture. Additional delays may be encountered to transport traffic between satellite hops. For a satellite link, a signal would be transmitted from a BTS to the satellite and from the satellite to the router, instead of via a T1 link. The time required to transmit from the BTS to the satellite of a particular orbit to the router is on the order of about 14 milliseconds, which comprises a large percentage of the 20 milliseconds variable delay limitation of the CDMA architecture, further which is greater than the remaining backhaul delay of approximately 8-9 milliseconds and thus satellite communication will be dropped. In other words, if the selector (SEL) does not see a response within the 20 milliseconds variable delay, then SEL drops the call.
Thus, the current CDMA system does not support having BTSs located on multiple scattered islands (for example, countries such as Indonesia) or isolated areas (such as deserts). This is largely due to the necessity of relying on backhaul satellite transport in such circumstances.
It would thus be desirable to provide an improved mobile communications system architecture for overcoming the problems as discussed herein above.
According to one embodiment of the present disclosure, a mobile communications system having a multi-level distributed frame selection and power control architecture includes a plurality of base station transceiver subsystems (BTSs) arranged in cells. Each base station transceiver subsystem (BTS) includes a capability for establishing a radio link or radio frequency link interface with a subscriber unit in conjunction with a telephone call. A PSEL provides for implementing a power control and frame selection of compressed packet data in conjunction with the telephone call, the PSEL coupled to and being positioned proximate the plurality of base station transceiver subsystems. A router is coupled to the PSEL for routing compressed packet data to and from the PSEL. Furthermore, a CSEL provides for implementing call processing and call management in conjunction with the telephone call, the CSEL coupled between the router and a prescribed mobile switching center (MSC), and further being positioned proximate the MSC, wherein the router is further for routing compressed packet data to and from the CSEL.
In addition, according to the embodiments of the present disclosure, the multi-level distributed frame-selection and power control CDMA architecture provide for the transmission of compressed voice data over the PSTN to advantageously reduce costs, in comparison with transmitting uncompressed voice data. This takes into account that the capacity of unchannelized T1 is significantly larger than channelized T1 when variable rate packets are transmitted over a T1 link. In addition, the present embodiments enable vocoder DSP units of a CDMA system to be grouped in one location to advantageously reduce a system operating cost and network blocking.
The embodiments of the present disclosure provide advantages which include, for example, eliminating the limiting effect of backhaul delay; providing a method for reducing the cost of operating with the use of T1 lines by increasing call capacity of a T1 line and reducing a number of required T1s for use in connection with soft handoffs; facilitating the transfer of compressed data as far as possible to reduce transport costs; performing a selection of frames before transmitting over T1 links; and increasing a CDMA router voice call switching capacity.